cmake: set CMAKE_CONF_INSTALL_DIR to lib/cmake/hiredis.

[cyfdecyf]

Installing cmake modules to <PREFIX>/lib/cmake/ is more common and make it easy for users to use hiredis package.

If backwards compatibility should be kept, maybe we can install to both share/hiredis and lib/cmake/hiredis.

[chayim]

@cyfdecyf In light of this PR I think this needs revisiting. WDYT?

[cyfdecyf]

I'll close this PR because find_package(hiredis) works with cmake configs files installing to share/hiredis.

I can't remember if I encounter any problem when I submit this PR. But seems the cmake doc about find_package search directory is right so share/hiredis can be used.